In a typical optical disk recording head, the objective lens is mounted in an electromagnetic actuator. This actuator is capable of translating the lens in two orthogonal directions; one for focusing and one for tracking. The focus direction is generally controlled by a continuous analog servo. Tracking, on the other hand, has a variety of modes available. One of these modes is always a track following mode, wherein the tracking servo-system utilizes some form of optical tracking error detection technique to control the tracking actuator such that the laser spot is held, centered over the track being followed. At times, the need arises to move the laser spot from the current track being followed to another track. If the destination track is close enough, it may be reached by moving only the objective lens (via the tracking actuator) as opposed to moving the entire head (via an access carriage). The operation of moving the objective lens from one track to another, is referred to as a track jump.
The actuator mechanically supports the objective lens with light flexures. These provide the desired freedom of movement in the focus and tracking directions and restrict unwanted twisting motions. A simple spring mass model is generally used to mathematically describe the actuators behavior in servo-system analysis and design. In this model the actuator driving current results in a proportional force on the mass (in addition to the spring force). For most purposes this model is adequate, but it does have some inaccuracies.
The track jump operation is generally performed by deactivating all tracking servo-systems, and sending a jump pulse waveform to the tracking actuator. This is done open loop because there is no feedback available during the jump. The shape of the jump pulse waveform is shown in FIG. 2(A). It consists of two parts, an accelerate pulse (1) which starts the lens moving in the desired direction, and a decelerate pulse (2) which slows the lens to a stop as it approaches the desired position. FIG. 2(B) shows the lens velocity along the tracking axis. FIG. 2(C) shows the lens position as it moves from the start track to the destination track. The exact shape of these curves may be calculated from the differential equation which describes a spring mass system, and the boundary conditions of the starting track location and the destination track location. This calculation is the approach which is generally used to determine the pulse duration times for pulse (1) and pulse (2).
In operation, a tracking actuator does not conform to the spring mass math model precisely, also some actuators behave differently than others. As a result, some track jumps don't end up on the desired track. This adversely affects the systems performance, because subsequent retry track jumps must be made. An object of this invention is to improve the accuracy of track jump pulse time selection and thereby the accuracy of the track jumps.
When a drive is new, all possible track jumps are calculated (by the conventional technique described above) and tabulated, by software, in a matrix as illustrated in FIG. 3. The track numbers are all relative to the center of the tracking actuators travel. Tracks outside the range of the matrix cannot be reached without moving the entire head assembly (the matrix is sized large enough to accommodate the full range of actuator travel). The parameters calculated and stored in the individual cells are the actual track jump pulse times for that particular jump. The matrix illustrated covers the whole range of possible jumps, organized by start track on the horizontal and destination track on the vertical. A subset of this matrix could be used instead; tabulating only the short jumps for instance. Once tabulated, in order to perform a jump, the drive simply looks up the pulse times in the matrix (for the jump it needs to perform) and outputs the track jump waveform.